An image display device (hereinafter referred to as a display) using liquid crystal elements, namely, a liquid crystal display is widely used for a monitor of various kinds of information equipment such as television receivers and personal computers, owing to its features of a thin structure and low power consumption. Particularly, the liquid crystal display for late mobile appliances is required to achieve higher image quality and higher resolution as well as to have added values such as improved functionality, a thinner and lighter structure, and reduced manufacturing cost. Lately, there has been an increasing trend in the development of what is called a system-in-display that incorporates drivers (drive circuits) for driving timing controllers and pixels, wherein the drivers are constructed with thin-film transistors (hereinafter also referred to as TFTs) using a low temperature polysilicon film on a glass substrate that is the same as for the liquid crystal display.
TFTs are already used to drive pixels in the liquid crystal display. Heretofore, an amorphous silicon material with low carrier mobility (hereinafter simply referred to as mobility) has been used. It is desirable to use a silicon material with higher mobility, because the TFT for driving the driver circuits is required to have high driving ability.
The most bottleneck for this requirement is the use of a large glass substrate in display production. For this reason, the temperature of the TFT fabrication process is restricted by the allowable temperature limit of the glass. However, in recent years, a technology that enables polysilicon crystallization and TFT fabrication on the glass substrate with a low temperature process not higher than 600° C. has been put into practical use.
At the present, as a method for crystallizing polysilicon at a low temperature, an excimer laser annealing (ELA) process is a mainstream. The ELA process irradiates an amorphous silicon film with high-output excimer laser pulses, thereby melting and re-crystallizing a large area of silicon film. Since this ELA process does not control the crystal growing direction, the orientation of crystal grain boundaries is random and the grain size is small in the order of 0.2 to 0.8 μm. The thus formed crystal grains have a large surface roughness with the swelling grain boundaries.
Since these grain boundaries limit the current driving performance of the TFTs and the reliability of the elements, a technique of crystallizing polysilicon more like a single crystal is considered. As an example of such technique, a Selectively Enlarging Laser Crystallization (SELAX) process is disclosed in a compilation of papers submitted to the Society for Information Display 2002 International Symposium (Boston, 2002) PP158-161. This method is to move (scan) the laser radiation on the silicon film formed on the substrate (or the stage on which the substrate underlying the formed silicon film is loaded) in a certain direction with regard to the substrate surface, using continuous-wave (CW) laser or pseudo CW laser light with a extremely high pulse frequency of several tens of MHz or higher.
By this laser scanning, the crystal grows along the certain direction. The crystal grain boundaries are formed substantially in parallel with the crystal growing direction. Consequently, no swelling takes place at the grain boundaries and, therefore, a film with a planar surface is obtained. The crystal grains are grown in a zone which is about 0.2-0.8 μm wide and about 2-8 μm long. This formation of the crystal grains introduces anisotropy between electrical conductivity in the crystal growing direction and that in the direction perpendicular to the growing direction. That is, the electrical conductivity in the crystal growing direction produces higher mobility, because of a decrease in the density of the crystal grain boundaries across which carriers pass.
As the CW laser light, for example, a light obtained by converting a fixed laser wavelength of 1064 nm into 532 nm is used. The output of the CW laser light or the pseudo CW laser light is smaller than the pulse excimer laser. Therefore, the laser light must be shaped into a beam for efficient crystallization. That is, the shape of the beam irradiating the substrate should be a long and thin rectangle with its long axis perpendicular to the laser scan direction. This is intended to increase throughput by enlarging a region to be scanned at a time. The long side of this rectangle is, in most cases, not more than 100 μm, though depending on the laser output. Thus, crystallization by annealing using the CW laser is selectively applied only to peripheral circuit portions where high-performance TFTs are needed.